Let the puff pastry reach room temperature. Preheat oven to 180ºC. Using a little bit of olive oil, coat the pan you’ll use to bake the tart. Spread the dough and with a knife, trim any excess. Prick the tart all over with the tines of a fork and refrigerate briefly to cool. When the tart firms, fill with pie weights and put in the oven for 15 minutes. Refrain from cooking thoroughly because you will finish the baking later on.
As the tart is baked, prepare the filling. In a pan, add the offal, onion, salt and pepper in medium heat, cover and let it cook for 15 minutes, until the onions become translucent. Add the spring onions, spinach, cherry tomatoes and cook for 10 more minutes, uncovered this time. Get the tart out of the oven and remove the pie weights. Fill the tart with the filling, making sure to avoid excess fluids. Add the kasseri and bake for 20 minutes to 180ºC. Allow the tart to cool before serving.

Tip: For a more piquant effect, make a mixture of feta and kasseri cheese. Served with a salad and you got yourself a complete meal.